The invention relates in general to liquid crystalline polyesters comprising the polycondensation product of an aliphatic dicarboxylic acid or a derivative thereof and a mesogenically substituted diol. More particularly, the present invention relates to liquid crystalline polyesters suited to be used as a medium for optical data storage, especially reversible (or erasable) optical data storage
A polyester of the type indicated above is known from European Patent Application 0,258,898, which discloses polyesters comprising repeating units of the following general formula: ##STR1## wherein: R.sup.1 is --H, --CH.sub.3 or --C.sub.2 H.sub.5,
l is an integer having a value of 1 to 20, PA1 k is an integer having a value of 1 to 30, PA1 A is --O--(oxygen) or --COO--, PA1 m is 0 or 1, PA1 and R.sup.2 is ##STR2## wherein Y is --COO-- or --OCO--, and R.sup.3 is COOR.sup.4, --OCOR.sup.4, --OR.sup.4, --COR.sup.4 or --R.sup.4 where R.sup.4 is ##STR3## R.sup.5 being --CH.sub.3, --CN or a halogen radical, n and p being the same or different and representing an integer having a value of 0 to 10, with the proviso that if R.sup.5 is --CH.sub.3, p is not 0, X being a halogen radical, q being 0 or 1, and C being an asymmetric carbon atom. PA1 n is an integer having a value of from 2 to 12, PA1 Z is CH.sub.2 or an oxygen atom, PA1 a+b is 2 to 4, with the proviso that both a and b are not equal to zero, PA1 A is --CH.sub.2 --CH.sub.2 --, --CO.sub.2 --, --O.sub.2 C--, --CH.sub.2 O--, --OH.sub.2 C--, --CH.sub.2 --CH.dbd.CH--CH.sub.2 --, --CH.dbd.CH-- or a single bond, PA1 B is --NO.sub.2, --CN, --OR, --R, --CF.sub.3, wherein R is a linear alkyl or branched alkyl having up to 10 carbon atoms,
The polyesters of EP 0,258,898 are used as display elements, more particularly for large screens and curved screens. With regard to possible use as a medium for optical data storage, however, the known polyesters display serious drawbacks, the most important of which is their low glass transition temperature.
For a material to be suitable as a medium for optical data storage its T.sub.g must be above room temperature. Information laser-written into a glassy polymeric material will not remain unimpaired at a temperature near T.sub.g on account of the too large rotational freedom of the polymer chains in the material
The above polyesters display T.sub.g values varying from -30.degree. C. to -9.degree. C. and are thus rendered unsuitable for use in an optical data storage medium.